Initial efforts in optical communications were concentrated in the transmission arena where optical fiber was used as a high bandwidth transmission medium in an otherwise electronic matrix. Electrical signals representative of intelligence were transformed into optical signals for transmission along an optical fiber and, at the receiving end, were transformed back into electrical signals for further processing. It was realized, however, that the cost of installing and operating the optical systems could be minimized if the signals could also be processed in optical form instead of only being transmitted in optical form. Accordingly, techniques for switching and processing optical signals are being developed. Among the first areas into which optical communications is moving, beyond transmission, is the field of optical amplification and regeneration.
In particular, optical amplifiers using erbium-doped optical fibers have recently become commercially available. These amplifying fibers can be coupled end-to-end to a transmission fiber and transversely coupled through a directional coupler to a laser diode pump. Such optical amplifiers exhibit low noise, relatively large polarization independent bandwidth, reduced cross talk and low insertion loss. In addition, they are relatively cheap to fabricate.
In optical transmission systems and networks, signal power levels may vary significantly due to variations in transmission path lengths and uncontrollable splitting and splicing losses. These variations of signal power can cause serious problems at an optical receiver whose design is normally optimized for a single signal power level.
A device for providing an optical signal having a relatively constant output power for optical input signals of varying power is disclosed in Electronics Letters, Jan. 31, 1981, Vol. 27, No. 3, pages 211-213, entitled "High Gain Limiting Erbium Doped Fiber Amplifiers With Over 30 dB Dynamic Range" by W. J. Way et al. The device disclosed is a optical limiting amplifier which has three separate stages of erbium doped optical fiber amplifiers positioned in tandem. Each stage of the assemblage must be separately pumped and filters are required between the various stages. This requirement of three separately pumped stages makes the device relatively expensive to fabricate.
A need exists for a more economical optical limiting amplifier that sets the output signal power to a predefined value for input signal powers of various values.